Use gluBeginTrim to mark the beginning
of a trimming loop, and gluEndTrim to mark the end of a trimming loop.
A trimming loop is a set of oriented curve segments (forming a closed
curve) that define boundaries of a NURBS surface. You include these trimming
loops in the definition of a NURBS surface, between calls to gluBeginSurface
and gluEndSurface.

The definition for a NURBS surface can contain many trimming
loops. For example, if you wrote a definition for a NURBS surface that resembled
a rectangle with a hole punched out, the definition would contain two
trimming loops. One loop would define the outer edge of the rectangle; the
other would define the hole punched out of the rectangle. The definitions
of each of these trimming loops would be bracketed by a gluBeginTrim/gluEndTrim
pair.

The definition of a single closed trimming loop can consist of multiple
curve segments, each described as a piecewise linear curve (see gluPwlCurve)
or as a single NURBS curve (see gluNurbsCurve), or as a combination of
both in any order. The only library calls that can appear in a trimming
loop definition (between the calls to gluBeginTrim and gluEndTrim) are
gluPwlCurve and gluNurbsCurve.

The area of the NURBS surface that is displayed
is the region in the domain to the left of the trimming curve as the curve
parameter increases. Thus, the retained region of the NURBS surface is inside
a counterclockwise trimming loop and outside a clockwise trimming loop.
For the rectangle mentioned earlier, the trimming loop for the outer edge
of the rectangle runs counterclockwise, while the trimming loop for the
punched-out hole runs clockwise.

If you use more than one curve to define
a single trimming loop, the curve segments must form a closed loop (that
is, the endpoint of each curve must be the starting point of the next curve,
and the endpoint of the final curve must be the starting point of the first
curve). If the endpoints of the curve are sufficiently close together but
not exactly coincident, they will be coerced to match. If the endpoints
are not sufficiently close, an error results (see gluNurbsCallback).

If
a trimming loop definition contains multiple curves, the direction of the
curves must be consistent (that is, the inside must be to the left of all
of the curves). Nested trimming loops are legal as long as the curve orientations
alternate correctly. If trimming curves are self-intersecting, or intersect
one another, an error results.

If no trimming information is given for a
NURBS surface, the entire surface is drawn.